The heart of a computer's long term memory is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider toward the surface of the disk and when the disk rotates, air adjacent to the surface of the disk moves along with the disk. The slider flies on this moving air at a very low elevation (fly height) over the surface of the disk. This fly height is on the order of nanometers. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic transitions to and reading magnetic transitions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
In a typical design the write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. This sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer, both of which can be made up by a plurality of layers. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned substantially perpendicular to the air bearing surface (ABS) and is relatively insensitive to applied magnetic fields. The magnetic moment of the free layer is biased parallel to the ABS, but is free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. Since θ is near 90 degrees at zero field, the resistance of the spin valve sensor (for small rotations of the free layer from 90 degrees) changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP pinned spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru or Ir. The thickness of the coupling layer is chosen so as to antiparallel couple the magnetic moments of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a net magnetic moment, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.
A current in plane (CIP) spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
The ever increasing demand for greater data rate and recording density has lead a push to develop current perpendicular to the plane (CPP) sensors which are uniquely suited to use in such systems. CPP sensors include both CPP giant magneto-resistive (GMR) sensors, which use an electrically conductive spacer layer such as Cu as well as tunnel magneto-resistive (TMR) sensors, which use a thin, electrically insulating barrier layer like Al-oxide. The CPP GMR sensor operates based on spin-dependent bulk and interface scattering of the electrons while the TMR sensor operates based on the spin dependent tunneling of electrons through the barrier layer.
In order to stabilize the free layer in CIP GMR, CPP GMR, or CPP TMR sensors against fluctuations due to thermal agitation and to prevent it from breaking up into domains it needs to be biased. One form of biasing a sensor is by using a hard magnetic biasing layer, typically a Col-xPtx or Col-x-yPtxCry alloy (x being between 10 and 35 atomic % and y between 0 and 15 atomic %) deposited onto a suitable underlayer material on both sides and about the same level as the free layer. Typically the hard magnetic bias layers also comprise a seed layer of Cr or CrX (X═Mo,Ti,V) on which the magnetic Col-xPtx or Col-x-yPtxCry material is deposited to achieve crystalline texture and sufficiently high coercivity. An insulating gap separates the free and the hard biasing layers in CPP sensors to prevent electrical shunting. The hard magnetic bias layer has sufficiently high coercivity and remanence so that its remanent moment magnetostatically biases the free layer in a direction substantially parallel to the ABS. The total moment of the hard bias layer is typically several times the magnetic moment of the free layer. The actual moment value depends on the hard bias material, its shape and separation from the free layer.
Another approach to biasing the free layer is in-stack biasing where an in-stack bias layer is separated from the free layer by a non-magnetic spacer layer. The free layer is stabilized magnetostatically by flux closure and is generally antiparallel to the biasing layer. The biasing layer is typically exchange biased to an antiferronmagnet to pin its moment in a desired direction parallel to the medium facing surface. Alternatively, the bias layer moment can be pinned by using a hard magnet. Yet another approach to biasing the free layer is to insert an orthogonal coupling layer between the free layer and the bias layer. The orthogonal coupling layer is a material that magnetically couples the free and the bias layer in substantially orthogonal directions. The biasing layer is typically exchange biased to an antiferronmagnet to pin its moment in a desired direction perpendicular to the medium facing surface.
One major problem with stabilizing the free layer by external sources such as hard-biasing, in-stack biasing, or biasing with a orthogonal coupling layer is that they increase the complexity of the device. Hard biasing and in-stack biasing rely on magnetostatic coupling with the free layer, which is difficult to control for small sensor sizes. In-stack biasing and biasing with an orthogonal coupling layer increase the sensor stack-height which make these approaches less suitable as gap sizes are reduced for higher recording densities.
Therefore, there is a strong felt need for a mechanism to generate a uniaxial magnetic anisotropy in the free layer which may either be sufficient to self bias the free layer in a direction substantially parallel to the medium facing surface of the sensor or to augment biasing from external sources such as hard biasing or in-stack biasing. As used herein substantially parallel means that the easy axis is closer to parallel than perpendicular to the ABS.